getResource DISTINCT pagetitle

<?php
/**
 * getResourceUnique
 *
 * DESCRIPTION
 *
 * This Snippet select distinct resource by name.
 *
 * USAGE:
 *
 * [[!getResourceUnique? &tpl=`childItem-prodotti-row` ]]
 *
 */

$limit = (isset($limit)) ? $limit : 10;
$showHidden = (isset($showHidden)) ? $showHidden : 1;
$featured = (isset($featured)) ? $featured : '';
$tpl = $modx->getOption('tpl',$scriptProperties,'childItem-prodotti-row');
$id = (int)$modx->getOption('id',$scriptProperties,17);



$c = $modx->newQuery('modResource');
$c->select('DISTINCT `modx_site_content`.`pagetitle`');
$c->where(array(
  'published' => true,
  'deleted' => false,
  'hidemenu' => $showHidden,
  'link_attributes' => $featured,
));
$children = $modx->getChildIds($id);



if (count($children) > 0) {
    $c->where(array(
        'id:IN' => $children,
    ));
}
$c->sortby('publishedon','DESC');
$resources = $modx->getCollection('modResource',$c);
//$resources = $modx->getIterator('modResource',$c);

$output = '';


var_dump($resources);
foreach($resources as $resource) {
 
   $output .= '<br /> test' . $resource->get('pagetitle');
 
}

return $output;

[2015-11-26 09:53:09] (ERROR @ /index.php) Error 42S22 executing statement: 
Array
(
    [0] => 42S22
    [1] => 1054
    [2] => Unknown column 'modx_site_content.pagetitle' in 'field list'
)

$c->groupby('pagetitle');

$c->select('DISTINCT pagetitle');